The majority of telephone sets in use today use an electromechanical ringer actuated by 20Hz a-c ringing signals on the telephone line. Due to the inherent inertia required to move the ringer arm as well as built-in control mechanisms, little trouble is experienced in false ringing generated by transient voltage surges on the telephone line such as experienced during pulse dialling and hook switch flashing.
More recently, electronic tone ringers have been developed which are also powered by the same a-c ringing signals on the line. These ringing signals are first rectified and filtered to produce a d-c supply voltage which is then used to drive an oscillator which in turn develops the ringing signals in a small loudspeaker located in the telephone set. In order to prevent transient voltage spikes from momentarily operating the tone ringer, a two-stage RC filter in conjunction with a gating control circuit, is commonly used to control the d-c supply to the ringer. However, such a circuit provides only marginal protection against such spikes. The protection can be improved by increasing the time delay prior to actuation of the ringer. However, with the existing circuit, this would not be possible without increasing the input capacitances of the circuit, which would then affect the operating conditions of the balance of the telephone set.